Add missing cell example for rank deficient

test/runtests.jl

@@ -34,6 +34,23 @@ form = DataFrame(Any[[0.1,0.3,0.5,0.6,0.7,0.9],[0.086,0.269,0.446,0.538,0.626,0.
     @test isapprox(coef(lm1), coef(lm2) .* [1., 10.])
 end
 
+@testset "rankdeficient" begin
+    # an example of rank deficiency caused by a missing cell in a table
+    dfrm = DataFrame([categorical(repeat(string.('A':'D'), inner = 6)),
+               categorical(repeat(string.('a':'c'), inner = 2, outer = 4))],
+               [:G, :H])
+    X = ModelMatrix(ModelFrame(Formula(nothing, :(1+G*H)), dfrm)).m
+    y = X * (1:size(X, 2)) + 0.1 * randn(MersenneTwister(1234321), size(X, 1))
+    inds = deleteat!(collect(1:length(y)), 7:8)
+    m1 = fit(LinearModel, X, y)
+    @test isapprox(deviance(m1), 0.28856700971719657)
+    Xmissingcell = X[inds, :]
+    ymissingcell = y[inds]
+    @test_throws LinAlg.PosDefException m2 = fit(LinearModel, Xmissingcell, ymissingcell)
+    m2p = fit(LinearModel, Xmissingcell, ymissingcell, true)
+    @test isapprox(deviance(m2p), 0.2859221258731563)
+end
+
 dobson = DataFrame(Counts = [18.,17,15,20,10,20,25,13,12],
     Outcome = categorical(repeat(string.('A':'C'), outer = 3)),
     Treatment = categorical(repeat(string.('a':'c'), inner = 3)))